Select the option that is related to the third number in the same way…

Select the option that is related to the third number in the same way as the second number is related to the first number and the sixth number is related to the fifth number.

16 : 8 :: 21 : ? :: 11 : 6

The logic followed here is :

16 + 4 = 20

then 20 ÷ 5 = 4

and 4 × 2 = 8

Same as

11 : 6

11 + 4 = 15

then 15 ÷ 5 = 3

and 3 × 2 = 6

Similarly

21 : ?

21 + 4 = 25

then 25 ÷ 5 = 5

and 5 × 2 = 10